1. Field of the Invention
The invention generally relates to improved canine parvovirus (CPV) vaccine formulations and diagnostic tests. In particular, the invention provides improved CPV vaccine formulations and diagnostic tests comprising newly emerging dominant CPV variants currently circulating in canine populations.
2. Background of the Invention
Canine parvovirus (CPV) is primarily an enteric pathogen that infects dogs, especially young dogs. Parvovirus infection is characterized by acute diarrhea, fever and leukopenia in dogs and puppies more than 4 to 5 weeks old, and myocardial disease in younger puppies. The mortality rate from the disease in unvaccinated dogs is very high. While vaccines against CPV are available, because CPV is a single-stranded DNA virus and has an extreme ability to mutate, the virus shows a remarkable ability to vary antigenically and thereby elude the immune protection afforded by vaccines. Thus, constant monitoring of the antigenic type and genotype of the causative agent is necessary.
CPV was first isolated in 1978 and was named “CPV-2” to distinguish it from parvovirus canine Minute virus (CMV or CPV-1). CPV-2 is generally believed to be a genetic variant of feline panleukopenia virus (FPV) or of the mink enteric virus (MEV), and is genetically and antigenically very closely related to parvoviruses that infect minks, foxes, raccoons, and other carnivores. The CPV capsid contains a single-stranded DNA genome of about 5200 bases with only two open reading frames, although at least four proteins are encoded due to alternative mRNA splicing. Parvovirus capsid is made up of two viral proteins (VP), VP 1 and VP2, with VP2 being the major immunogenic parvovirus capsid protein. A genetic variant of the original CPV isolate was identified in 1979-1980 and was named CPV type 2a. In the mid-1980's, yet another variant, type 2b, was identified, and since then, types 2a and 2b appear to have completely displaced the original CPV-2. Current vaccines are directed against only the 2a and 2b variants, yet the 2a variant is no longer detected in the United States. For a review of CPV discovery and evolution, see Parrish and Kawaoka, 2005.
CPV-2b differs from CPV-2a at only two amino acid positions: Asn-426 in 2a (encoded by AAT) is Asp in 2b (encoded by GAT), and Ile-555 in 2a is Val in 2b. The Ile-555 to Val change is actually a reversion to the original type 2 sequence. The CPV-2a and 2b antigenic types appeared to be relatively stable for a number of years. However, in 2000 a variant “2c” was described in which position 426 is Glu encoded by GAA (position 555 remains Val). The 2c variant has been reported in Italy (Buonavoglia et al., 2001), Vietnam (Nakamura et al., 2001) and other countries, including Spain (Nakamura et al., 2004; Decaro et al., 2006), but until now there have been no confirmed reports of the 2c variant in the United States, and CPV vaccines have not been updated to include such variants. This is of special importance because, unlike previously described variants that infect primarily puppies, CPV2c has the ability to infect adult dogs. Further, the sequence of a 2b variant with codon variations at positions 494 and 572 was submitted to GenBank (gi: 54646340) in 2003 and reported by Shackelton et al. in 2005 (Proceedings National Academy Sciences 102:379-384), but no alterations in CPV vaccine or diagnostic compositions were proposed based on such sequences. This may be in part due to the nature of the mutations, which are “silent” with respect to the encoded amino acids, i.e. the translated amino acid sequence of 2b and the 2b variant with codon variations at positions 494 and 572 are the same. However, recent studies concerning codon bias (Enserink, 2008. Science 320: 1709; Coleman 2008, Science 320; 1784-87) show that codon bias may be a very important evolutionary tool of organisms. Enserink states that “ . . . many organisms, including viruses, have a bias toward certain codons in their genes. This may be because those codons are easier to translate at ribosomes . . . thus speeding up protein production . . . ”. Therefore, to successfully combat diseases for which viruses are the etiological agent, it is advisable to continuously monitor emerging variants and include prevalent variants in new, updated vaccine preparations, even if the changes that are detected do not result in a change in a translated amino acid sequence.
Several problems arise when vaccines are not updated. Firstly, even vaccinated dogs may be susceptible to infection by CPV variants that are not included in the vaccine. Secondly, when vaccinated dogs become ill, their owners frequently claim that the viral strains in the vaccine caused the disease. This has frequently resulted in the payment of remuneration to the owners, since there is no practical way for the vaccine company to provide evidence to the contrary.
Several CPV vaccine preparations have been proposed:
U.S. Pat. Nos. 4,193,990 and 4,193,991 to Appel et al., describe heterotypic and inactivated (“killed”) virus vaccines, respectively, in which the exemplary vaccine afforded protection against the original CPV.
U.S. Pat. No. 4,303,645 to Carmichael et al., describes a vaccine comprising an attenuated CPV produced by prolonged serial passage of the virus in non-oncogenic cell lines. The exemplary vaccine also protects against the original CPV isolate.
U.S. Pat. No. 4,971,793 to Wood et al., and U.S. Pat. No. 5,882,652 to Valdes et al., both describe recombinant subunit vaccines comprising the CPV VP-2 protein produced in recombinant baculovirus. The VP-2 protein is of no specified type or subtype.
U.S. Pat. No. 5,885,585 to Parrish et al., describes a CPV vaccine comprising an attenuated form of a 2b variant.
Due to the ability of the CPV virus to mutate and develop new antigenic variants, there is an ongoing need to monitor the genetic makeup of CPV variants and to develop vaccines and diagnostic tests that reflect current CPV variants.